1. Field of the Invention
The invention concerns a device for receiving dual polarized microwave signals, constituted by simple elements that allow rapid and easy assembly without the need for subsequent adjustment, and which confer upon said assembly sufficient strength.
This device is more particularly intended for receiving dual polarized microwave signals emitted for television receiver sets, for example, from geostationary satellites. It is intended to be utilized in receiving stations, associated to the active elements equipping the microwave heads of said stations.
More generally, the device can be utilized for receiving all dual polarized microwave signals, by adapting certain of its elements to the frequency range to which it is intended.
A signal, or a wave, is called a dual polarized signal, or wave, when it is not propagated on one plane, as in the case of a rectilinear biased signal (called a plane wave), but about an axis determining the direction of propagation of the signal. This is the case with circular polarized waves, or elliptical polarized waves.
A dual polarized wave can be considered as being two superimposed rectiliner polarized waves.
The polarization is circular when the amplitude of the field vector (electric or magnetic) resulting from superimposing of the two waves is constant about the axis of propagation; more specifically, the end of the field vector forms a circle about the axis, when projected on a plane perpendicular to this axis.
A circular polarized wave results from two plane waves, that are orthogonal between each other, and the maximum amplitudes of which, as well as the frequencies, are equal, but between which a phase shift of 90.degree. exists.
The polarization is elliptical when the amplitude of the field vector varies around the axis of propagation. Its end forms an ellipse, when projected along a plane perpendicular to the axis.
An elliptically polarized wave results from two plane, orthogonal waves, the maximum amplitudes of which are different.
The circular polarization is principally utilized in satellite tracking stations or in space communications installations. The consequences of the Faraday effect that exists in the ionosphere are thus reduced, and reception is improved. Circular polarization is also utilized in ground-based radar systems, in order to limit the noise echoes due to clouds.
2. Prior Art
In order to exploit upon reception a wave emitted in with dual polarization, it is known to break up this wave into two plane waves, then to analyse each of the components obtained.
This means that a device for receiving dual polarized microwave signals or waves must comprise means for ensuring the reception and processing of two rectilinear polarized microwave signals.
In known devices for receiving rectilinear polarized microwave signals, the signal is picked up by using a probe or sensor that supplies the input of a microwave head.
The microwave heads comprise a pre-amplifier stage connected to the sensor, in order to amplify the signal received in the band of centimetric waves. A heterodyne converter, constituted by a local oscillator and a mixer, allows to transpose the frequency of the signal received by the sensor towards a lower frequency, for example in the band of decimetric waves; thereafter, an amplifier acts upon the transposed signal prior to its exploitation.
These various components comprise microstrip or stripline elements distributed on dielectric substrates having a thickness more or less important, depending upon whether they are associated to the decimetric waves circuit or to the centimetric waves circuit. Conductor sections are achieved through metallization on these substrates and their width varies according to the frequency band to which they are intended.
The sensor to pick up the signal is, in these different circuits, formed through using a metallization placed upon one of the substrates, and is positioned inside a waveguide connected to the receiving antenna.
The realization of a device for receiving dual polarized microwave signals presents drawbacks since this device should be equipped with two sensors and two substrate assemblies.
It is first of all necessary that the sensors be disposed so as to receive signals that are orthogonal to one another. It is thereafter necessary that the circuits supplying the microwave heads do not interfere; it is also necessary that assembly be rapid, and requires no subsequent adjustments, while presenting sufficient stiffness.